The present invention relates to methods of cooling, and in particular to a method of cooling an induction-heated vapor deposition apparatus.
Induction-heated vapor deposition apparatus typically includes a deposition enclosure (such as a epitaxial quartz tube or a bell jar) having disposed therein a susceptor-supported wafer and disposed thereabout the essentially electrically uninsulated turns of an RF induction heating coil. As current is passed through the coil turns, the coil induces heating of the susceptor to a temperature (at least 500.degree. C.) sufficient to cause the gas passing through the deposition enclosure to deposit epitaxially on a wafer.
Unfortunately, as the susceptor heats the wafer, it also heats the deposition enclosure, thereby promoting the deposit of the gases passing through the deposition enclosure upon the interior walls of the deposition enclosure. Such deposits on the interior walls of the deposition enclosure have a deleterious effect on the quality of the material being grown within the deposition enclosure, promoting the development of spikes, pits and various other surface defects. Such deposits may furthermore include dopants from a given run, in which case the deposits may act in future consecutive runs as an unwanted source of the dopants. In order to remove these deposits once they have formed, the deposition enclosure must be taken off stream for cleaning, thereby lowering the productivity of the deposition apparatus.
In order to prevent such deposits from forming, it is customary to cool the deposition enclosure by any of a variety of known techniques. For example, air, nitrogen or other gases may be directed in a cooling stream onto the exterior of the deposition enclosure to absorb and remove heat therefrom. A water jacket may be disposed about the deposition enclosure, the water flowing therethrough acting to absorb and remove heat from the exterior surface of the deposition enclosure. The deposition enclosure may be at least partially surrounded by a black box which absorbs radiant heat from the exterior surface of the deposition enclosure. An exhaust system such as a shroud-encompassing tube may be used to remove heat from the immediate vicinity of the exterior surface of the deposition enclosure. However, none of the known cooling techniques provide sufficient cooling without unduly increasing the capital and operating costs of the system (for example, by requiring expensive gas cooling, heat exchangers, exhaust ducts and the like).
To fully comprehend the complexity of the problem, it must be appreciated that the cooling technique used must not only avoid any possibility of a short circuit between the ends of the RF coil (a typical voltage differential of about 16 kilovolts exists at this point), but also even a short circuit between adjacent turns of the RF coil or from one turn to ground. There is a basic matching between the oscillator circuit of the RF induction source (that is, the generator) and the coil induction frequency of the susceptor within the deposition enclosure. Any frequency change from the normal (typically about 4.5 megahertz, but dependent upon the equipment used) due to mismatching as a result of minimal shorting or grounding of coil turns, even within the one millivolt range, results in a loss of tuning of the oscillator and thus an effective loss of power.
Accordingly, it is an object of the present invention to provide a method for cooling an induction-heated vapor deposition apparatus which is both economical and efficient.
Another object is to provide such a method which is safe and easy to operate.
A further object is to provide apparatus for use in such a cooling method.